This invention relates generally to in place regeneration of an adsorbent such as activated carbon using microwave energy and more particularly concerns microwave regeneration of a portion of a carbon bed while the rest of the bed is still actively online.
In industry, process streams carrying contaminants or other components are often purified by passing the stream in contact with an adsorbent. The contaminants or other components are adsorbed by the adsorbent, thereby removing them from the process stream. Adsorption is most effective when the adsorbent is maintained at ambient temperatures or cooler. The adsorbed materials are referred to as adsorbates or simply sorbates. Thus, a sorbated adsorbent refers to an adsorbent having adsorbed materials therein. In the course of cleansing process streams, the adsorbent will eventually become saturated with sorbates and be unable to adsorb further materials. Rather than simply being disposed of, a saturated adsorbent can be recycled through a process which desorbs or strips the sorbates from the adsorbent. Once the sorbates have been desorbed, the adsorbent is again capable of being used to cleanse process streams.
Such processes are generally referred to as regeneration because they regenerate or renew the adsorbing capacity of the treated adsorbent. In the case where the adsorbent is activated carbon, a distinction is made sometimes where low temperature processes (i.e., in the range of 200.degree.-400.degree. F.) are referred to as regeneration and higher temperature processes (up to 1800.degree. F.) are referred to as reactivation. However, for the sake of clarity, the term "regeneration" as used herein, will include both low and high temperature desorbing processes. It is desirable to employ a regeneration process which is capable of stripping the sorbates on the plant site, because such a process eliminates the need to ship the sorbated carbon off site for cleaning. Besides offering cost advantages, on site regeneration reduces the number of plant emissions which must be reported to the Environmental Protection Agency.
A typical method of regenerating a saturated adsorbent is to heat the adsorbent with a flow of hot gas such as steam, nitrogen or flue gases to a sufficiently high temperature at which the sorbate will be desorbed. The high temperature causes the sorbated matter to vaporize and pass from the adsorbent. The flow of the hot gas also purges the vaporized or desorbed materials from the system. The adsorbent bed must be taken offline to be swept with the hot gas. Some disadvantages of this gas heating method include long regeneration times, large amounts of purge gas, nonuniform heating of the adsorbent material, dilution of the sorbate vapors with heating gases, and generation of sorbate condensates diluted by a large fractions of water if steam is used as the heating gas. Furthermore, the gas heating method requires heating not only the adsorbent material but also the entire structure containing the adsorbent, which is necessarily several times heavier than the adsorbent. Thus, the design of the containment structure is controlled by the temperature and corrosion limits prescribed the the regeneration process.
Microwave heating of the adsorbents has been proposed to avoid some of the problems associated with the hot gas heating method. Microwave heating has an advantage in that the adsorbent material alone can be heated without directly heating the containment structure. By heating only the adsorbent, the energy required for regeneration is reduced. The cost of the containment structure can also be reduced since the structure is subjected to lower temperature ranges. Microwave heating also can produce generally higher regeneration temperatures than the gas heating method; for example, steam regeneration is usually limited to about 250.degree. F.
A simple approach to microwave heating is to transfer the adsorbent from the adsorber vessel to a bulk container and expose the container to microwave energy in order to heat the adsorbent to the regeneration temperature. However, this approach is still inefficient and time consuming, because it is a non-continuous or batch operation, wherein the adsorbent bed is not only taken offline but removed from the adsorber vessel for transfer to the bulk container. Besides creating excessive downtime and transfer expense, the required handling of the adsorbent material can result in heavy attrition losses. Handling the adsorbent is made even more difficult when treating adsorbent containing water, dirt and/or other solids due to agglomeration of adsorbent granules.